Future Day is my favourite, simply for incredible title track, it's pure improvisational ambient spacerock.
But you can't go wrong with Tago Mago and Ege Bamyasi.
Another good starting point is Cannabilism 1 which is a great anthology and contains Mother Sky (from Soundtrack) - the track The Horrors beautifully ripped off with Sea Within A Sea.
Truly exceptional band - and one whose influence is ever present in today's big hitters.

with what the average reply seems to be and say either Tago Mago or Ege Bamyasi. I started with Tago Mago myself, pretty much purely because it seems to be the most often mentioned and it has the - absolute - 'Mushroom' on it.